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Archive for the ‘nuclear physics’ Category

The OPAL neutron reflector “leak”.

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Australian Greens senator Scott Ludlam has again this week called for Australia’s 20 MW OPAL research reactor to be closed down, following reports that a minor problem with the neutron reflector in this “tank-in-pool” reactor has yet to be rectified.

The facility has been out of operation for 11 of the past 14 months, during which time Australia has had to rely on costly imports of medical and scientific radionuclides from foreign suppliers in South Africa and Canada.

Schematic diagram of the OPAL core and neutron reflector
(Thanks to ANSTO for these images. Click through for large high-resolution images.)

The OPAL reactor core sits in the centre of a heavy water neutron reflector, which itself sits within the reactor’s large pool of light water, as we see in the above diagram.

In the centre of the circular heavy water vessel is the nuclear fuel itself, an array of 16 fuel assemblies. The large and small holes that pass through the entire height of the reflector, into the reactor core, support the generation of products such as transmutation-doped silicon and medical and scientific radionuclides as well as supporting neutron irradiation experiments. Several different neutron beamlines are also installed into the reactor, set up for different neutron spectra, including a liquid deuterium moderated cold neutron source.

Here, the square reactor “core” is clearly visible in the centre of this photograph, illuminated strongly by its own Čerenkov radiation, with the round neutron reflector surrounding it, pierced by the aforementioned ports for the irradiation of samples, with the greater reactor pool, containing light water, surrounding that.

The purpose of the neutron reflector is to improve neutron economy in the reactor, and hence to increase the maximum neutron flux – neutron flux being a fundamentally important metric of the performance and usefulness of a research and isotope production reactor.

To maximise the neutron flux or neutron economy in the reactor, heavy water, being a good moderator, basically a material from which elastic scattering of neutrons readily occurs, is used to construct a neutron reflector, immediately surrounding the reactor.

You’ve got light water from the pool seeping into the heavy water neutron reflector that surrounds the reactor. So, the light water from the pool is “leaking” into the reactor components, in towards the reactor. The reflector vessel is kept at a lower pressure than the light water at ambient pressure in the reactor pool. Any leakage pathway at all will allow light water to seep into the reflector vessel, diluting the heavy water. This issue was first identified in December of 2006, following commissioning of the new reactor, and attempts have been made to address the problem during an extended shutdown, which have been somewhat, but not totally, successful.

The sole consequence of this is that it dilutes the expensive heavy water. Of course, some people, and some media reports, seem to persist in documenting such a “leak” as though it were luminous green radioactive goo tricking out into suburban Lucas Heights.

If the heavy water is diluted to any significant extent, the efficiency of the neutron reflector is diminished, and the neutron flux that is achieved under nominal operating conditions is diminished, making the reactor less efficient for neutron beam experiments, neutron irradiation or radionuclide production. There is absolutely nothing here of any health physics or safety significance, at all, period. This dilution of the heavy water in the reflector vessel has absolutely no significance with regards to safety of the facility.

The Greens have derided ANSTO’s comments on the nature of the fault as “spin” and link these technical concerns to some kind of supposed, imaginary potential for safety concerns in the future. Of course, Ludlam wouldn’t know what a neutron reflector was if it bit him, and he has a proven track record of carrying on fervently about issues of nuclear science and technology, whilst possessing an alarming lack of understanding of such science and technology; especially for a federal politician.

Once the heavy water in the vessel becomes diluted, the only way to un-dilute it is via the same methods of deuterium enrichment such as those originally used to make it – such as distillation, or the Girdler sulfide process. In the case of a high deuterium concentration, as in a tank of somewhat diluted heavy water, distillation is the best option. Apparently, ANSTO are planning to construct a small-scale heavy water re-distillation system for online re-enrichment of some of the heavy water passing through the reflector circulation loop. This will fully counteract the problem, and allow the use of the reactor with the fullest efficiency for research and isotope production.

Nuclear Australia has got more to add about this issue, and ANSTO’s response to media reports and the Greens’ misleading statements is to be found here.

Anyway, Senator Ludlam and the Greens are not just content with calling for the reactor to be shutdown until the heavy water dilution issue can be rectified or nullified, however – they are quite adamant in calling for the permanent shutdown of the reactor.

“We think the safest solution for this reactor is for it to be shut down and for the waste to be contained properly,” Greens senator Scott Ludlam said this week. Importing radionuclides from international suppliers such as in South Africa and Canada could continue, he said.

In addition to the production of medical radionuclides, the reactor is used to produce neutron-transmutation-doped silicon boules for microelectronics – a valuable commercial service marketed by ANSTO – as well as for the production of radiopharmaceuticals and scientific radiochemicals. The radionuclides, most of them employed in nuclear medicine, typically commonly produced with ANSTO’s reactor, are thus:

Samarium-153 – 1.93 days
Molybdenum-99 – 2.75 days
Indium-111 – 2.83 days
Iodine-131 – 8 days
Chromium-51 – 27.8 days
Iodine-125 – 59.4 days

Half-lives are as indicated. The short half-life of 153Sm, the basis of the onocological radiopharmaceutical Quadramet, in particular means that importation of this radionuclide is difficult and impractical, and it is essentially unavailable in the absence of an operating isotope production reactor in Australia.

We’ve learned from painful experience that the supply of expensive imported radionuclides has been subject to delays or interruptions to supply during shutdowns of OPAL (and HIFAR) in the past. On the basis of ANSTO’s past experience, it can reasonably be assumed that still worse problems would arise if Australia were to be totally reliant upon imported radionuclides. The supply problems arise from a range of causes, such as weather delaying flights, aviation regulations relating to radioisotopes being carried with other goods, or opposition from freight pilots.

The International Atomic Energy Agency has identified the “growing problem of refusal by carriers, ports and handling facilities to transport radioactive material” as a significant problem for nuclear medicine and scientific research involving radionuclide importation across the world, and has initiated processes intended to identify ways in which it can be overcome. A number of international, such as British Airways, no longer accept carriage of radioactive material, and others have imposed tight restrictions. Unless a way can be found to reverse such trends, shipments of radionuclides across the world will become increasingly problematic.

The reactor and its associated neutron guides and instruments are used for neutron radiography, neutron scattering imaging, neutron reflectometry and other advanced neutron-beam based research and technological applications, neutron activation analysis, for example for forensic applications, as well as the analysis and testing of materials under neutron irradiation and research into the potential for Boron Neutron Capture Therapy as a potent weapon against cancer – which requires the patient to be bought to a nuclear reactor to produce the thermal neutron flux required.

Even if some radionuclides can be imported, clearly our research reactors in Australia are of significant importance and usefulness in such fields. If radionuclides are to be imported from foreign suppliers, they are still being produced in similar nuclear reactors – if a research reactor is such a dangerous thing, as is suggested by these groups, why should foreign nations be subjected to such a burden for the production of radiopharmaceuticals which are for the benefit of us? Why shouldn’t we take responsibility for our own reactor, if we have decided that we value the benefits of its products, and we’re not prepared to forgo them?

The “Georeactor” Hypothesis.

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This post was inspired, in part at least, by Rod Adams’ post on NEI Nuclear Notes recently, asking about the georeactor theory. I hope you find it useful, Rod.

The “georeactor” hypothesis is a proposal by J. Marvin Herndon that a fissioning critical mass of uranium may exist at the Earth’s core and indeed that it serves as the energy source for the Earth’s magnetic field. You can read all about Herndon’s ideas at his website.

Herndon’s georeactor hypothesis is not widely accepted at all by the scientific community, outside of Herndon himself and a very small number of defenders.

Herndon’s georeactor hypothesis is sometimes confused with the existence of natural nuclear fission reactors in the Earth’s crust in rich uranium deposits at Oklo in Western Africa – however, it must be stressed that these are not the same thing – there is absolutely no doubt at all, scientifically, as to the occurrence of nuclear fission and the formation of natural nuclear “reactors” at Oklo approximately two billion years ago.

However, Rob de Meijer and associates at the Nuclear Physics Institute in Groningen, the Netherlands, are amicable towards Herndon’s theory, and have indeed proposed an experiment by which it should be somewhat falsifiable – involving measurement of the antineutrino flux from the Earth’s core which they believe will validate the georeactor hypothesis.

Fission reactors generate huge numbers of electron antineutrinos – about 10^26 per day from a typical manmade power reactor. Several thousand of these can be measured per day in a detector of modest size, outside the reactor, outside the containment, tens of meters away.

The antineutrinos resulting from each fission event from uranium and plutonium have different total count rates and energy spectra – the antineutrinos are not actually produced by nuclear fission itself, but rather by the beta decay of fission products. The antineutrinos therefore carry with them information about the amount and type of fissile material in the reactor core, and the rate at which it is being fissioned.

Because of this, incidentally, the use of neutrino detectors has raised considerable interest in recent times in the context of providing a real-time online and very simple measurement of the fuel burnup, operating status, power level, plutonium production and such characteristics of operating nuclear reactors, which is of considerable utility in enforcing non-proliferation safeguards.

(There’s more information on this application here if you’re interested.)

Personally, I don’t see why existing underground neutrino observatories, such as Super-Kamiokonde, the Sudbury Neutrino Observatory, and the IceCube experiment in Antarctica shouldn’t be sufficient to provide significant insights into the presence – or absence – of georeactor antineutrinos. Clearly all neutrinos from a “georeactor” come exclusively from exactly the centre of the earth as observed at every detector, and they should be detectable at all neutrino observatories worldwide with a similar flux everywhere.

Combining these simple pieces of information with the expected energy spectra of neutrinos from uranium fission, it seems extremely plausible that the georeactor hypothesis can well and truly be put to the test, using existing experiments, and probably even with existing collections of raw data from these experiments.

As one of Herndon’s recent papers puts it:

Uranium, being incompatible in an iron-based alloy, is expected to precipitate at a high temperature, perhaps as the compound US. As density at Earth-core pressures is a function almost exclusively of atomic number and atomic mass, uranium, or a compound thereof, would be the core’s most dense precipitate and would tend to settle, either directly or through a series of steps, by gravity to the center of the Earth, where it would quickly form a critical mass and become capable of self-sustained nuclear fission chain reactions.

Of course, there is what seems like one significant problem with this theory – whilst several billion years ago, the portion of uranium-235 in natural uranium was much higher than it is today – equivalent to that of manmade enriched uranium, because U-235 decays faster than U-238, although a much larger ratio of U-235 was originally formed when the uranium was formed inside supernovae than is seen in the Earth today. That is why fission occurred at Oklo two billion years ago, but does not occur today – there is not enough of a concentration of U-235 in nature. Therefore, how can a “georeactor” exist?

Herndon explains away this question by postulating that the georeactor is something like a fast breeder reactor, started up aeons ago when the U-235 was more abundant, and today burning the abundant U-238 into plutonium-239.

However, if this is the case, couldn’t it be likely that we could observe plutonium-fissioning “breeder reactors” in rich uranium deposits in the Earth’s crust, like at Oklo, today?

Hans Bethe on Fission-Fusion Hybrid Reactors

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Well, this is my light reading for tonight – from Physics Today, May 1979.

Very interesting stuff indeed – and of course, there’s no doubting that the author knows what he’s talking about.

Written by Luke Weston

March 28, 2008 at 6:00 pm

Fast spectrum reactors and supercriticality.

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I’ve been reading this little essay on the history and progress of India’s nuclear energy programs recently. I encourage you to have a look, if you’re interested.

It’s a bit critical of India’s interest in Breeder reactor technology, suggesting that fast-neutron-spectrum “breeder” reactor technology has been tried, and rejected, throughout most of the nations of the world with significant experience in nuclear engineering.

There’s one particularly interesting claim made here:

“If the operating system failed to insert control rods fast enough, the increased reactivity would, in turn, heat up the sodium further; this chain could ultimately cause a fuel meltdown into a supercritical configuration and a small nuclear explosion. “

Helen Caldicott’s Nuclear Madness also says essentially the same thing:

“Once out of control, a fission reaction in a breeder could cause not only a meltdown but also a fully fledged nuclear explosion.”

A sufficiently enormously supercritical configuration would indeed have the potential to be the makings of a nuclear explosion – that’s how a nuclear fission bomb works, by compressing fissile material together into a massively supercritical mass.

Technically, a supercritical mass is any mass of fissile material with an overall neutron multiplication ratio, k > 1. k < 1 represents an ordinary, sub-critical mass, and k = 1 represents a, just marginally, critical mass.

k > 1 in the core of a fission reactor represents an unrestrained, massive power excursion – which usually results in the explosion of the reactor, if this condition is sustained for any significant length of time.

Such an event results in a significant release of thermal energy – in the presence of a water coolant, the resulting rapid production of steam pressure contributes significantly to the explosive force of such an event.

A significant amount of direct neutron and gamma radiation is released from such a critical assembly.

Such power excursions were responsible for the destruction of the reactor in the Chernobyl reactor disaster and the destruction of the experimental SL-1 reactor in the US in 1961.However, in all practical cases, it is not easily conceivable that the neutron multiplication factor would be great enough, and that the fissile material would stay together long enough, without destroying itself, and without reducing neutron multiplication via thermal expansion or doppler broadening, for a fully fledged nuclear weapon-style to occur. This seems to me to be quite implausible.

Any nuclear engineers out there care to share their thoughts?

Written by Luke Weston

August 20, 2007 at 1:31 am